Studies show that the MIMO technology improves the multiplexing gain and diversity gain of a communications system significantly. Based on a Bell-labs Layered Space Time (BLAST) technology (which is a time-space communications technology patented by Bell Labs), different transmitting antennas transmit different data streams. In this case, the system capacity increases in proportion to the order of the MIMO channel. Based on a space-time coding and single-beam forming technology, all antennas transmit the same data stream to improve transmission reliability. The maximum diversity gain is equal to the product of the number of the transmitting antennas and the number of receiving antennas. Nevertheless, depending on the specific requirements on the rate and reliability, the MIMO system may be designed to utilize both multiplexing gain and diversity gain, namely, achieve a tradeoff between multiplexing and diversity.
In a Frequency Division Duplex (FDD) system, downlink channels are hardly available, and the downlink beam formation vector is generally calculated by a mobile terminal, and is selected from a codebook of a limited length. The serial number of the best codeword selected is fed back by the mobile terminal to a Base Station (BS) through a low-rate feedback channel. Such a method is generally known as limited feedback precoding.
Unlike FDD, a TDD system is characterized by reciprocity between downlink channels and uplink channels because uplinks and downlinks use the same frequency resource. That is, a downlink channel gain may be obtained through estimation of the uplink channel gain. Based on such channel information, multiple precoding technologies may be adopted. For example, Singular Value Decomposition (SVD), Zero Forcing (ZF), Minimum Mean Square Error (MMSE), Tomlinson-Harashima Precoding (THP), and Vector Precoding (VP). For a TDD MIMO system, the prerequisite of using the foregoing precoding technologies is that the mobile terminal needs to use all antennas to transmit signals, so that the BS obtain all information of the MIMO channel. However, due to power consumption of the power amplifier and complexity, currently the mobile terminal still uses a single antenna to upload signals although it can receive signals through multiple antennas. Therefore, by using the reciprocity, the BS obtains only Partial Channel State Information (PCSI) corresponding to one antenna of the mobile terminal. Evidently, the traditional precoding technology is not realizable on the basis of only such PCSI. In this case, the precoding technology needs to be implemented on the basis of PCSI and channel statistic information.
A precoding implementation method in the prior art is implemented in a TDD MIMO system through a Pseudo-Eigen Beam (PEB) forming technology. The conception of the precoding implementation method is to reconstruct a correlation matrix of the channel, and then select beams of multiple streams through SVD. The reconstructed relevant matrixes are made up of three weighted parts, namely, a correlation matrix which is made up of instantaneous PCSI on the BS side and has a rank of 1; a long-term channel correlation matrix fed back by the mobile terminal; and long-term statistics of unitary spaces which are selected randomly on the BS side and include instantaneous PCSI.
However, such a method involves feedback of long-term statistics from the mobile terminal, and requires SVD performed by the BS and the mobile terminal simultaneously in order to select the transmission mode. Meanwhile, the BS needs to maintain a random statistic, which is implemented through continuous Quadrature Right-triangle (QR) decompositions. Therefore, the implementation is rather complicated. Moreover, no best weight value is determined in the PEB forming technology, and an empiric weight value needs to be used in practice. Evidently, the PEB forming technology does not make full use of the precise PCSI, and the corresponding data stream rate is not ensured.